Field of Invention
The invention relates to a control system of a vehicle, especially a tractor, equipped with a Continuously Variable Transmission (CVT) of the hydrostatic-mechanical split type which includes a hydraulic drive circuit in which a hydraulic pump supplies pressurised fluid to a hydraulic motor.
Description of Related Art
A hitch, such as a three-point linkage is one known arrangement used to attach implements to a drawing vehicle, for example an agricultural tractor. The implement may be fully mounted or semi-mounted on the tractor whereby a semi-mounted implement has a wheel engaging with the ground during soil operation while a fully-mounted implement puts all its load on the three-point linkage.
Three point linkages most frequently consist of two lower lifting arms to which an implement is attached. The lower lifting arms can be pivoted by respective hydraulic actuating cylinders to adjust the height position of the implement relative to the tractor. Furthermore, these lower lifting arms may be manually adjusted by length to be appropriate for an implement to be attached. An additional top link connects the implement to the tractor above the lower lifting arms. This top link is used to pivot the implement about a horizontal transverse axis and is adjustable by means of a threaded connection, or a hydraulic cylinder.
Alternative designs of three-point linkages are known, such as the arrangements shown in U.S. Pat. No. 6,321,851, US 2003/217852 and U.S. Pat. No. 5,997,024, in which the lower links are replaced by two or four variable length hydraulic rams. This variable length ram arrangement enables multi axis movement of any implement attached to the linkage.
To control the three-point linkage, modern tractors are mainly equipped with electronic linkage control systems to improve work quality and operator comfort during operation.
Such electronic linkage control systems operate in three well known modes:                Position control: In general, the tractor speed is kept constant by a speed control system and the position of the lower lifting arms is sensed directly or indirectly so that the working depth of the implement in the soil can be adjusted whilst the speed of the tractor is kept constant.        Draft control: The implement is raised and lowered in the soil depending on the draft force applied by the implement to reduce fuel consumption, avoid engine stall or avoid damage of the implement or tractor. Again, vehicle speed is kept constant. If the implement is lowered into the ground an initial draft is applied defining a zero level. The operator can then set a value representing a force increase which means that the operator can decide how fast the implement is lifted when a small force increase or a large force increase occurs. The value of the force entered by the operator does not represent an exact value of the force applied, e.g. 5 kN, but defines the responsiveness of the draft control. The objective of this function is to move the implement while avoiding excessive draft or pull force variations. Therefore, a draft force sensor, typically in the form of a draft force sensing pin which connects the lower lifting arms to the tractor chassis is used to measure the horizontal load applied to the tractor by the implement.        Intermix of position/draft control: This control arrangement, as its name implies, is a mixture of position and draft control in which a draft control system can only lift the implement within a limited range of positions. This function is provided to avoid excessive movement of the implement in the soil resulting in poor working quality. Again, vehicle speed is kept constant by a speed control system.        
Only the draft control and intermix mode (both referred to as drag modes) operate under measurement of the drag force. Generally, deactivating the drag modes results in that the system enters the position mode with no drag force influencing the lifting heights. It may however be difficult to install a draft force sensing pin due to the complex three-dimensional geometry of a linkage. Further, the sensing pins may become dirty or damaged and thus may not function properly. Accordingly, a control system which does not rely on sensing pins is preferred.
A linkage control based on CVT parameters can result in that the control system moves the position of the linkage over a wide vertical displacement range as a reaction to the drag force. Various situations have been identified in which the drag mode should be deactivated since otherwise the draft control will cause the implement to crash to the ground, or cause the linkage to collide with the wheels of the tractor, or the drawbar of an trailer.
Detecting these situations can be quite difficult. Some implements are simply towed by attachment to a ball hitch rather than being mounted to the linkage, with actuators on the implement controlling operating conditions of the implement based on information received from the tractor, for example via a CAN-BUS link, or ISOBUS.
The following are situations, where if the drag mode is not deactivated, the movement of the linkage in response to the drag force may cause damage to the tractor or implement and/or be dangerous.
1. In the case in any tractor operational mode, the higher the speed of the tractor, the more dangerous is the consequence of an unintentional automatic movement of the implement in drag mode.
2. In the case where an implement is attached to a ball hitch on the tractor, or a when a tow bar is attached to the tractor, that is the linkage is not used, a drag force determined by the CVT would deliver a significant change of drag signal when the roll of the tractor changes, or the vehicle travels uphill, or downhill or during acceleration, or when coasting. Under normal circumstances, this would cause the linkage to move, and thus when a tow bar is attached would cause it, or the linkage to collide with the tow bar.3. In the case where the tractor is reversing, or coasting, the change in drag signal would ordinarily cause the linkage to move.4. In the case where the vehicle is at a stand still, the tractor speed is zero and therefore the drag mode is not deactivated. However, if the drag mode is then activated, the linkage is automatically lowered.5. In the case where an implement is being transported in a lifted position, the operator is ordinarily responsible for deactivating the draft or intermix mode manually when travelling along a road with an implement held in a lifted position. If this is not done the drag force determined by the CVT delivers a significant change of drag signal when the roll of the tractor changes, or the vehicle travels uphill, or downhill or during acceleration, or during coasting. This could result in the implement to be lowered and crashing to the ground.6. In the case where an implement is attached which is not in contact with the ground during operation, for example fertiliser spreaders and sprayers, the CVT delivers a significant change of drag signal when the roll of the tractor changes, or the vehicle travels uphill, or downhill or during acceleration. This would ordinarily result in the implement being raised which is not intended, or lowered which is also not intended and may be dangerous if not expected.7. In the case when a tractor brake is activated during driving, the change in drag force may result in the linkage moving.